Cyanidation is a well known process for extracting gold and silver from their ores. The process involves grinding the ore and then leaching the ore in the presence of oxygen with a mixture of lime and cyanide, which results in the dissolution of the gold and silver values as the dicyanospecies viz., Au(CN).sub.2.sup.- and Ag(CN).sub.2.sup.-, respectively. The precious metal values may be recovered from the leach liquors by adsorbing the values onto activated carbon. Adsorption may occur directly from the cyanided pulp as in the carbon-in-pulp process, or from solution, where fluidized or fixed bed contactors containing granular carbon are generally employed. Since the amount of gold that can be extracted onto the carbon is strongly dependent on the initial concentration of gold in solution, and is in equilibrium with the concentration of gold remaining in solution (see FIG. 1), high gold loadings on the carbon cannot be achieved from typical leach liquors or secondary mining effluents e.g., Merril tails or damreturn water.
Other disadvantages of conventional carbon-in-pulp and carbon-in-leach circuits for the recovery of precious metal and other cyanides are:
The operational problems experienced with interstage screening. PA1 During the many hours carbon resides in a circuit, it becomes fouled physically, or poisoned reducing its capability for recovering desired species. PA1 Having to accept either low recovery or relatively low loadings and thus eluate tenors of some species when attempting co-recovery of multiple species which have isotherms that differ significantly. PA1 Large carbon inventories are needed to compensate for the drop in rate of adsorption as carbon approaches saturation relative to the isotherm in each stage. PA1 The many air inlets for sweeping interstage static screens and for interstage airlifting introduce oil, CO.sub.2 and O.sub.2 which can all be deleterious to the process.
Furthermore, the adsorbed gold values are difficult to strip from the carbon, and rapid and efficient elution with a caustic-cyanide solution can only be achieved at elevated temperatures, typically greater than 90.degree. C. Conducting the elution under pressure or modification of the eluant by addition of organic compounds also has a beneficial effect on the rate of elution, but these processes are more complicated to implement.
Generally, the conventional processes have required the need for relatively large quantities of eluant leading to large volumes of dilute electrolyte.
There is also a risk of mixing substantial quantities of acid and cyanide in the elution equipment and consequent generation of hazardous HCN in the event of the failure of certain valves.
The adsorption of dissolved gold cyanide onto activated carbon is represented by Equation (1), ##STR1## which indicates that adsorption of gold onto the carbon is highly favoured. If the dicyano-gold species in the carbon can be converted into a species that is insoluble in the aqueous loading medium, the reaction represented in Equation (1) will be driven to the right, resulting in a further increase in the ability of the carbon to load gold. Thus treatment of the carbon so as to promote the reaction represented in Equation (2), EQU Au(CN).sub.2.sup.- .revreaction.AuCN.sub.(insoluble) +CN.sup.31 ( 2)
may be expected to enhance the ability of activated carbon to extract gold.